3. •Food serves as a source of raw materials for the cells in
the body and as a source of energy.
•Cells break down food into simpler molecules in a
process that releases energy to power cellular activities.
Animal Cells
Animal
Mitochondrion
Plant
Plant Cells

4. Harvesting Chemical Energy
• Cellular respiration is the process by which cells
break down organic compounds (food) to
produce ATP.
• Both autotrophs and heterotrophs perform
cellular respiration.
• The primary fuel for cellular respiration is
glucose, which is formed when carbohydrates
such as starch and sucrose are broken down.
• If too few carbohydrates are available, other
molecules, such as fats and proteins can be
broken down to make ATP.

5. Harvesting Chemical Energy
• When cells break down food molecules,
some of the energy in the molecules is
released as heat.
• The remaining energy is stored in
molecules of ATP.
• ATP is the energy “currency” of cells.

6. Adenosine Triphosphate
• ATP supplies energy for 3 main types of
biological work:
– Mechanical functions of cells
– Active transport of ions and molecules across
cell membranes
– Synthesis and breakdown of large molecules
• The phosphate groups of ATP store energy.
• This energy is released when the bonds
that hold the phosphate groups together
are broken.
• The cell uses this energy to do work.

7. The ATP-ADP Cycle

8. Overview of Cellular Respiration
Electrons carried in NADH
Electrons carried
in NADH and
Pyruvic FADH2
acid
Glucose Glycolysis
Cytoplasm
Mitochondrion

9. Overview of Cellular Respiration
• Cellular respiration is the process that
releases energy by breaking down glucose
and other food molecules in the presence of
oxygen.
• The equation for cellular respiration is:
6O2 + C6H12O6 → 6CO2 + 6H2O + Energy
oxygen + glucose → carbon dioxide + water + Energy

10. Overview of Cellular Respiration
• Glycolysis takes place in the cytoplasm. The
Krebs cycle and electron transport take place in
the mitochondria.
Glycolysis
Cytoplasm
Mitochondrion

11. Glycolysis
• ATP Production
− At the beginning of glycolysis, the cell uses
up 2 molecules of ATP to start the
reaction.
2 ATP 2 ADP 4 ADP 4 ATP
Glucose
2 Pyruvic
acid

12. Glycolysis
− When glycolysis is complete, 4 ATP
molecules have been produced.
2 ATP 2 ADP 4 ADP 4 ATP
Glucose
2 Pyruvic
acid

13. Glycolysis
− This gives the cell a net gain of 2 ATP
molecules.
2 ATP 2 ADP 4 ADP 4 ATP
Glucose
2 Pyruvic
acid

14. Glycolysis
• NADH Production
− One reaction of glycolysis removes 4 high-
energy electrons, passing them to an electron
carrier called NAD+.
2 ATP 2 ADP 4 ADP 4 ATP
Glucose
2NAD+ 2 Pyruvic
acid

15. Glycolysis
− Each NAD+ accepts a pair of high-energy
electrons and becomes an NADH molecule.
2 ATP 2 ADP 4 ADP 4 ATP
Glucose
2NAD+ 2 Pyruvic
2
acid

16. Glycolysis
− The NADH molecule holds the electrons until
they can be transferred to other molecules.
2 ATP 2 ADP 4 ADP 4 ATP
2NAD+ 2 Pyruvic
2
acid
To the electron
transport chain

17. Glycolysis
• The Advantages of Glycolysis
− The process of glycolysis is so fast that
cells can produce thousands of ATP
molecules in a few milliseconds.
− Glycolysis does not require oxygen.

18. Fermentation
• When oxygen is not present, glycolysis is
followed by a different pathway. The combined
process of this pathway and glycolysis is
called fermentation.
• Fermentation does not require oxygen—it is an
anaerobic process.
• Fermentation does not produce ATP, but it does
regenerate NAD+, which allows for the continued
production of ATP through glycolysis.

19. Fermentation
• There are many types of
fermentation. Two of the most
common types are:
– Alcoholic fermentation
– Lactic acid fermentation

20. • Alcoholic Fermentation
− When oxygen is not present, yeasts and certain bacteria use
alcoholic fermentation, forming ethyl alcohol and carbon dioxide
as wastes.
− The equation for alcoholic fermentation after glycolysis is:
pyruvic acid + NADH → alcohol + CO2 + NAD+
− Yeasts, added to crushed grapes, eat the grapes’ sugars and
produce wine when there is no oxygen present.
− Yeasts, added to the grain barley, eat the grain’s sugars and
produce beer when there is no oxygen present.

21. • http://www.youtube.com/watch?
v=mqioniPbEHQ
• http://www.youtube.com/watch?
v=JfsQEVDxuFI

22. • Alcoholic Fermentation
− The CO2 released by the yeast causes the
carbonation of some alcoholic beverages, such as
champagne and beer.
− Yeasts, added to dough, digest sugars (derived
from starches in dough) and produce carbon
dioxide, causing the dough to rise.

23. • Lactic Acid Fermentation
− In some cells, pyruvic acid that accumulates as a
result of glycolysis can be converted to lactic acid
when oxygen is not present.
− This type of fermentation is called lactic acid
fermentation. Like alcoholic fermentation, it
regenerates NAD+ so that glycolysis can continue.
− The equation for lactic acid fermentation after
glycolysis is:
pyruvic acid + NADH → lactic acid + NAD+

24. • Lactic Acid Fermentation
– During strenuous exercise, oxygen is scarce;
therefore, human muscle cells switch from
aerobic respiration to lactic acid fermentation.
 Lactic acid that accumulates as a waste
product may cause muscle soreness, but it is
gradually carried away by the blood to the
liver.
 Lactic acid is converted back to pyruvic acid
by liver cells.

25. • Lactic Acid Fermentation
− Lactic acid fermentation by certain fungi
and bacteria is used in the dairy industry
to make cheese and yogurt.
 Milk bacteria digest the milk sugar
lactose and produce lactic acid, which
acts with the added enzyme rennet to
curdle the milk. The cheesemaker
drains off the whey and compacts the
curds, which various microbes then
ripen into a mature cheese.
− Lactic acid fermentation is also used to
make pickles and sauerkraut.
 The cucumbers and cabbage are soaked
in a salt brine and sealed, allowing the
growth of bacteria that eat the vegetable’s
sugars and produce tart-tasting lactic
acid.

26. • http://www.youtube.com/watch?
v=d0UfS1bqscM

28. • The first part of the equation is glycolysis.

29. • The second part shows the conversion of
pyruvic acid to lactic acid.

30. Efficiency of Glycolysis
• After glycolysis, only about 2% of the
energy contained in glucose has been
transferred to ATP.
• Most of the energy is still stored in pyruvic
acid.
• Large organisms that require more energy
will meet their needs by undergoing
aerobic respiration.

32. Overview of Aerobic Respiration
• After glycolysis, if oxygen is present,
fermentation will not occur. Instead, 2
pathways called the Krebs cycle and the
Electron Transport Chain will occur.
• Because these pathways of cellular
respiration require oxygen, they are aerobic.
• Aerobic respiration produces nearly 20 times
as much ATP as is produced by glycolysis
alone.

33. • Both plant and animal cells carry out the Krebs
cycle and the Electron Transport Chain in the
mitochondria.
Animal Cells Outer membrane Intermembrane
Mitochondrion
space
Inner
membrane
Matrix
Plant Cells

34. Overview of Aerobic Respiration

35. The Krebs Cycle
• Discovered by Hans Krebs in
1937
• He received the Nobel Prize in
physiology or medicine in 1953 for
his discovery.
• Forced to leave Germany prior to
WWII because he was Jewish
• The Krebs cycle occurs in the
mitochondrial matrix.
Mitochondrial
Matrix

37. The Krebs Cycle
• During the Krebs cycle, pyruvic acid is
broken down into carbon dioxide in a series
of energy-extracting reactions.

38. – The Krebs cycle
begins when pyruvic
acid produced by
glycolysis enters the
mitochondrion.

39. – One carbon atom
is removed,
forming CO2, and
electrons are
removed,
changing NAD+ to
NADH.

40. – Coenzyme A joins
the 2-carbon
molecule, forming
acetyl-CoA.

41. – Acetyl-CoA then
adds the 2-carbon
acetyl group to a 4-
carbon compound,
forming citric acid, a
6-carbon compound.
Citric acid

42. – Citric acid is broken down into a 5-carbon
compound, then into a 4-carbon compound.

43. – Two more molecules of CO2 are released and
electrons join NAD+ and FAD, forming NADH and
FADH2. NADH and FADH2 carry high-energy
electrons and are referred to as “Electron Taxis”.

44. – In addition, one molecule of ATP is generated.
– However, the Krebs cycle turns twice (once for
each pyruvic acid molecule produced in
glycolysis)!

45. The Krebs Cycle
• Totals after the Krebs Cycle:
– 2 ATP
– 8 NADH (plus 2 NADH from glycolysis = 10 total NADH)
– 2 FADH2
– 6 CO2
• Most of the energy released in the breakdown of glucose
still has not been transferred to ATP.
• The 10 NADH molecules and the 2 FADH2 molecules drive
the next stage of aerobic respiration where most of the
energy transfer occurs.

46. Electron Transport Chain
• The electron transport chain (or ETC) uses the
high-energy electrons from NADH and FADH2 to
convert ADP into ATP by moving protons down
their concentration gradient (chemiosmosis).
– The ETC is located in the inner mitochondrial
membrane in folds called cristae.
Inner
Mitochondrial
Membrane

47. – High-energy electrons from NADH and FADH2
are passed along the electron transport chain
from one carrier protein to the next.

48. – At the end of the chain, an enzyme combines
these electrons with hydrogen ions and oxygen
to form water.

49. – As the final electron acceptor of the electron transport chain, oxygen
gets rid of the low-energy electrons and hydrogen ions.
 The Importance of Oxygen
 ATP can only be produced if electrons keep moving down the electron transport
chain
 Without oxygen to accept electrons, the electron transport chain stops and no more
ATP can be produced

50. – When 2 high-energy electrons move down the
electron transport chain, their energy is used to
move hydrogen ions (H+) across the membrane.

51. – During electron transport, H+ ions build up in the
intermembrane space, so it is positively charged.

52. – The other side of the membrane, from which those
H+ ions are taken, is now negatively charged.

53. – The inner membranes of the mitochondria
contain proteins called ATP synthases.
ATP
synthase

54. – H+ ions move down their concentration gradient
through channels into the ATP synthase. This
causes the ATP synthase to spin.
Channel
ATP
synthase

55. – As it rotates, the enzyme grabs a low-energy ADP,
attaching a phosphate, forming high-energy ATP.
Channel
ATP
synthase
ADP

56. Electron Transport Chain
Animation

57. Electron Transport Chain
Song

58. The Totals
• Glycolysis produces just 2 ATP molecules per
molecule of glucose.
• The complete breakdown of glucose through
aerobic cellular respiration, including glycolysis,
results in the production of 36 molecules of ATP.
− Efficiency of Cellular Respiration
 This represents about 37% of the energy
stored in glucose.
 The remaining energy is released as heat.

59. The Totals

60. Comparing Aerobic & Anaerobic
Cellular Respiration Pathways
Aerobic Anaerobic
(needs (no oxygen)
oxygen)
Occurs in: Most Mostly yeast
organisms and bacteria
1 glucose 6 CO2 + 6 H2O Ethanol + CO2
makes: or lactic acid
Net ATP 36 2
production:

61. Comparing Photosynthesis
and Cellular Respiration
• The energy flows in photosynthesis and
cellular respiration take place in opposite
directions.

62. Photosynthesis-Cellular
Respiration Cycle

63. Comparing Photosynthesis
and Cellular Respiration
• On a global level, photosynthesis and cellular
respiration are also opposites.
– Photosynthesis removes carbon dioxide
from the atmosphere and cellular respiration
puts it back.
– Photosynthesis releases oxygen into the
atmosphere and cellular respiration uses
that oxygen to release energy from food.